Rework Supply chain chapter: solution does not handle errors properly because of the specifics of the files API

[greenfork]

The currently listed solution is:

use "files"

actor Main
  new create(env: Env) =>
    try
      let dir = FilePath.mkdtemp(FileAuth(env.root))?
      let writer = TempWriter(dir, "free-candy.txt")
    else
      env.err.print("Couldn't create temporary directory")
    end

actor TempWriter
  let _file: File

  new create(dir: FilePath, file_name: String) =>
    _file = File(dir)

  be record(it: String) =>
    _file.write(it)

The constructor create creates a file descriptor but does not check for errors, the correct code to check for errors would be:

  new create(dir: FilePath, file_name: String) =>
    _file = File(dir)
    if not (_file.errno() is FileOK) then error end

But it is not possible to do so because an actor constructor cannot fail, and actually defeats the purpose of this chapter because the chapter advises to do error handling before constructing an actor.